Ultimate Load Strength Method working load and compared with permissible stresses. Ultimate Load method and Limit State method for RC members subjected to flexure. To design RCC Structures, Working stress method, load factors, or ultimate load method, limit state method points are considered. ULS Method is based on the Strength concept which is affected, working loads are increased by multiplied them with the load factors to give the ultimate loads.
Ultimate Load Strength Method Limit State Method
The load factor gives structure, proper and adequate safety against failure. stated that the Load factor is the ratio of ultimate strength to the service loads. In the Ultimate load strength method, stress condition at the state of an impending collapse of the structure is analyzed, thus using the non-linear stress-strain curves of concrete and steel.
Ultimate Load Strength Method
The safety measure in the design is obtained by the use of proper load factor. under combined loading conditions different load factors are used. Ultimate load method used the real stress-strain curve of concrete and steel and takes into account the plastic behavior of these materials. it gives an exact margin of safety in terms of load, unlike working stress method which is based on the permissible stresses which do not give any idea about the failure/collapse load.
Ultimate Load Strength method was accepted by IS CODE 1964 as an alternative to the working stress method. The structural element is proportioned to withstand the Ultimate Load which is obtained by enhancing the service load by some factor referred to as load factor for giving a desired margin of safety.
Working Stress Method:
Working Stress Method main assumption on the behavior of structural material is restricted with the linear elastic region and the safety of it is ensured by restricting the stresses coming on the members by working loads. The stresses in the materials, which are developed due to working loads are restricted giving a safe design. Working Stress method will lead to large FOS and over-sized sections, thus the sections will be uneconomical stress is directly proportional to strain up to the point of collapse. Based on the elastic theory, and assuming that the bond between steel and concrete is perfect, permissible stresses of the materials are obtained. effect of creep, shrinkage, stress concentrations, and other secondary effects the assumption of material behavior in the elastic range will not hold on the considered point of the yield of steel.
Limit State Method
Limit State method is based on the concept of safety and serviceability. Limit state is a condition just before collapse. Limit state Structure designed by limit state should give proper strength and serviceability throughout its life. Limit state of serviceability- Limit state of serviceability deals with control on deflection, cracking, vibration and corrosion. The design of members is initially done by considering the limit state of collapse and the members is checked for serviceability requirement after the design.
Comparison Between Working Stress Method and Limit State Method
Working stress method base on an elastic theory which assumes that steel and concrete are elastic and the stress-strain curve is linear for both
The limit state method is base on the actual stress-strain curves of steel and concrete. For concrete the stress-strain curve is non-linear.
Working Stress method assumes that the actual loads, permissible pressures, and factors of safety have been understood. So it’s called a deterministic method.
This way is based upon the probabilistic approach that depends upon the real data or expertise, thus it’s referred to as a non-deterministic method.
In the working stress method, the material follows Hooke’s law as stress is not allowed to cross the yield limit.
Limit state method, stress is allowed to cross the yield limit.
Tags: Limit State Method, Work Stress Method